The present application claims priority of Italian patent application No. IT SV2001A000039, filed on Oct. 31, 2001. The entire contents of which are incorporated herein by reference.
The invention relates to a circuit for active decoupling of transmit coils from receiving coils in Nuclear Magnetic Resonance imaging apparatuses, particularly of the low field type.
Systems of this type are currently known, which allow active decoupling, i.e., based on the feeding of a transmit coil control signal. The control signal consists of an adequate bias current which is provided to a PIN diode to switch the latter to the conductive state during transmission, or which is suppressed to switch the PIN diode to the nonconductive state, hence to generate the transmit coil cutoff condition in which said transmit coils are decoupled from the receiving coils during reception of echo signals, with respect to mutual induction interference caused by induction.
Transmit coils are decoupled to prevent the latter from generating noise in the reception of echo signals by the receiving coils, due to mutual induction interference between said transmitting and receiving coils.
The cutoff of transmit coils prevents any current from being induced in the transmit coils during reception, which current might generate an influence on the receiving coils.
Two types of decoupling circuits are basically available at present, i.e. passive and active decoupling circuits.
In passive decoupling circuits, the conductors of transmit coils have at least two silicon diodes, for automatic cutoff, i.e., decoupling of the transmit coils from the receiving coils when no RF coil exciting signal is present. This type of decoupling circuit has certain drawbacks. First, since the excitation pulses consist of oscillating currents, when the level of these pulses falls below a certain threshold, the diode is switched to an open-circuit condition, in which the transmit coil is cutoff and decoupled. The conductive state is restored when the signal level rises again above the diode conduction threshold. This behavior introduces distortions in transmit coil exciting pulses, which affect image quality, due to abnormal excitation of transmit coils and the consequent deformations in the RF signal generated by the coils. Therefore, the quality with which image slices are selected is also affected. This condition is generally further worsened by the fact that transmit coils are composed of several sections connected in series, one diode or one pair of diodes being associated in series with each of said sections. Each of these diodes clips the excitation pulse based on the value of its own conduction threshold.
The above clearly shows that, theoretically, the passive decoupling circuit has a threshold-based operation, while the decoupling of transmit coils requires a decoupling circuit that has a time-based operation, i.e., that is switched to the conductive state, in which transmit coils are not cut off, at the start of excitation pulse transmission on the feedline, and to the decoupling state, in which transmit coils are cut off, at the end of excitation pulse transmission, without introducing any signal deformation. Therefore, passive decoupling circuits provide approximations of the functions required from the decoupling circuit, which are based on the strengthening of silicon diode features.
Active decoupling circuits also use diodes, particularly PIN diodes, i.e., diodes whose conduction features may be controlled by an adequate bias current.
Although these PIN diodes are theoretically fit for the required functions, the conductive state being entered by feeding an adequate bias current, they still have a drawback. Since the coil exciting pulses consist of oscillating currents, in the negative half-period of said pulses, the diode bias charge, due to the bias current applied thereto, is reduced, and this may cause the bias condition to drop below the level required to keep the PIN diode in the conductive state. This drawback is associated with the period of the transmit coil exciting pulse. Referring to Nuclear Magnetic Resonance imaging apparatuses, which include devices for decoupling transmit coils from receiving coils, two categories shall be assumed to exist: high field apparatuses and low field apparatuses.
In high field apparatuses, the negative half-periods of excitation pulses have a short duration, hence the drawback of active decoupling circuits, operating with said PIN diodes, is removed or has a lower effect.
However, in low field apparatuses, the negative half-periods of transmit coil exciting pulses must be longer, hence PIN diode behavior becomes an important problem. In fact, in order that the PIN diode may be held in the conductive state for the long negative half-periods of transmit coil exciting pulses, very high bias currents should be applied, higher than those allowed for PIN diodes. When the duration of the negative half-period is such that the bias current applied is not sufficient to maintain the conductive state, the PIN diode gradually passes to the open-circuit condition. As the bias charge decreases, the internal resistance increases, and at high intensities of the transmit coil exciting current during the negative periods has a high intensity, the Joule value generated in the diode rises, and may reach such values as to cause the diode to be destroyed and/or the brazing tin seam on the track of the printed circuit to melt.
U.S. Pat. No. 5,621,323 discloses a decoupling circuit for receiving coils which includes two diodes, more precisely a fast and low-power PIN diode, and a conventional slow and high-power diode, connected in antiparallel in the circuit of the receiving coils. As used herein, xe2x80x9cantiparallelxe2x80x9d means in parallel, but with reversed polarities.
The decoupling circuit so formed is a passive decoupling circuit, which means that no currents for controlling the bias condition of the PIN diode are provided, and this circuit is proposed to avoid the use of active decoupling circuits. The patent expressly recommends not to use active decoupling circuits for receiving coils. Furthermore, from a functional point of view, the arrangement proposed in the patent, which does not relate to transmit coils, but only to receiving coils, in which induced currents are considerably smaller than in transmit coils, is based on a problem that is totally different from the one that forms the subject of this invention and does not relate to the bias current increase required to maintain the PIN diode in the conductive state.
EP 1 130 413 discloses the decoupling of receiving coils for Nuclear Magnetic Resonance apparatus clearly of the high field type.
In first instance the decoupling circuit comprises two PIN diodes arranged in antiparallel but the operation of the decoupling circuit for the receiving coil operates in a opposite manner as the decoupling of a transmission coil.
Furthermore the decoupling circuit disclosed in EP 1 130 413 does not consider the problems caused by the intensity of the bias current and by the power which can be dissipated by the pin diode which may considerably increase the costs of the circuits. These problems are not dramatic in combination of receiving coils but are dramatic in the transmitting coils.
Transferring the decoupling circuit of EP 1 130 413 from the receiving coil to a transmitting coil of an high field MRI apparatus has no use since the known active decoupling circuits formed by simple PIN diodes not connected to an antiparallel further diode are able to maintain the conduction of the RF current of the excitation pulse, by feeding to the PIN diode a reasonable DC bias current.
The transfer of a decoupling circuit according to EP 1 130 413 to a low field MRI apparatus is not obvious because the solution of the problem consists only in increasing the bias current and the power that the PIN diode may dissipate up to the desired values taking into account the increases of costs related therewith. EP 1 130 413 suggests using the disclosed decoupling circuit (page 4, line 40) for low power conditions, which is exactly the contrary of the situation to which the present invention applies, namely low field and high power conditions, thus rendering the above mentioned transfer of the decoupling circuit according to 1 130 413 to this condition is not obvious.
An object of the invention is to provide a circuit for active decoupling of transmit coils from receiving coils of a low field magnetic resonance apparatus by cutting said transmit coils off, while allowing to limit the bias current and to maintain the conductive state of the decoupling circuit even for long negative half-periods of excitation pulses, and obviating the drawbacks of the above mentioned prior art circuits.
The invention solves the above problem by providing a decoupling circuit like the one described hereinbefore, which uses at least an additional silicon diode connected in antiparallel to the PIN diode.
The decoupling circuit may include several PIN diodes connected in series to each other and/or to sections of the transmit coil conductor, each having an input for a bias current, which input is connected to a generator of said bias current, a silicon diode being provided for each of said PIN diodes and being connected thereto in antiparallel.
In accordance with the above description, the decoupling circuit of this invention has at least one pair of diodes connected in antiparallel with each other and in series with the conductor of the transmit coil, one of the two diodes being a PIN diode and the other a silicon diode, or two, three or more of said pairs of diodes being connected in series with each other and/or with the conductor of the excitation pulse transmitting coil.
Particularly, in combination with a transmit coil, including a certain number of turns, which are electrically separated from each other into several sections, connected in series with each other and with the excitation pulse feedline, there are provided, upstream from each coil section, one or more antiparallel pairs of PIN diodes and silicon diodes.
The silicon diode connected in antiparallel with the PIN diode allows to limit the bias current and to obviate the above drawbacks since, if the bias charge is reduced in the negative half-period of the excitation pulses, the PIN diode function will be substantially assumed by the silicon diode. In this condition, the intensity of the excitation pulse is rather high, certainly above the conduction threshold of the silicon diode and allows to obtain substantially distortion free excitation pulses.